Toll-like receptors (TLR) currently comprising a gene family of 10 receptors with different specificities are part of the cellular pathogen pattern recognition system, which has evolved for defense against a variety of infections (bacteria, virus, fungi). Activation of TLRs leads to cytokine responses, e.g. with release of interferons and activation of specified immune cells. The functional expression of selected TLRs in tissues is highly different. Part of the receptors are located at the cell surface such as TLR4 (stimulated by E. coli lipopolysaccharide LPS), e.g. on epithelial cells, or TLR3, 7, 8 and 9 located at endosomal membranes in specified immune cells. The latter are all activated by nucleic acids, but recognize various types of them. For instance, TLR9 is activated by single stranded DNA containing CpG subsequences, TLR7 and 8 are activated by single stranded RNA, and TLR3 is activated by double-stranded RNA.
Some small-molecule (SMOL) TLR7 or TLR8 agonists have been identified. Those agonists can be grouped into purine-like molecules, such as 7-thia-8-oxoguanosine (TOG, isatoribine) or the imidazoquinoline imiquimod. Imiquimod is so far the only approved definitive TLR agonist, marketed as 5% créme (by Aldara). It generates approx 80% 5 year clearance of superficial basal cell carcinomas, which is the most frequent cancer worldwide. Imiquimod activates TLR7. The functional expression of TLR7 appears to be restricted to specified immune cells, i.e. in humans so far solely plasmacytoid dendritic cells, B-cells and probably eosinophils are known to be activated by TLR7 agonists.
For several years strong efforts are ongoing worldwide trying to exploit the strong immune activation induced by TLR7, 8 or 9 agonists for the treatment of cancer. Cancer immunotherapy, however, experienced a long history of failures. In recent years, though, the knowledge on cancer immune surveillance and the function of subsets of immune cells thereby was improved drastically. TLR7 or TLR9 agonists are in clinical development for cancer mono- or combination therapies, or as vaccine adjuvant.
The TLR agonist approach for cancer immunotherapy is different from earlier efforts using, e.g. cytokines, interferons or monovalent vaccinations. TLR agonist mediated immune activation is pleiotropic via specified immune cells (primarily dendritic cells and B-cells, subsequently other cells), which generates an innate and adaptive immune response. Moreover, not only one interferon is induced, but rather the many different isoform's altogether, and not only type I (alpha, beta), but also (indirectly) type II (gamma, NK cells). At least for local application, Aldara has delivered a remarkable proof-of-concept. This demonstrates that antigens are released by tumors, and that immune therapy can work for cancer indications in principle, and even in monotherapy. For a systemic administration route, though, the clinical POC is pending for TLR7 or TLR9 agonists, which both are in clinical trials. For advanced cancers and systemic application (preferably s.c. or i.v. administration route) it appears to be clear that such TLR agonists have to be combined with other therapies.
In case of earlier stages of cancer the situation might be different. Tumor metastasis is a most severe aspect of tumor development in patients, largely because tumors are detected too late when metastasis already has occurred. Established tumor therapies mostly include cytotoxic drugs with rather narrow therapeutic windows. Hence, for the treatment in earlier tumor stages, when the suppression of metastasis spread might still be possible, the need is high for new therapies with good tolerability and safety.
The activation of the immune system, and in particular, the activation of toll-like receptor (TLR) signaling offers new promising approaches. TLR9 agonistic CpG oligos like H2006 or H1826, and TLR7 agonists like the guanosine derivative isatoribine or an imiquimod derivative were tested in our murine Renca lung metastasis model. All tested molecules virtually completely suppressed the emergence of lung metastases with good tolerability. This provides a convincing rational for clinical development of such molecules for suppression of cancer metastasis and points to the possibility of systemic application of such drugs. However, the SMOL type TLR7 agonists have the advantage of established and cost effective synthesis if compared to the nucleic acid type TLR9 agonists, and are well suited for topical application.
U.S. Pat. No. 6,573,273 describes imidazoquinoline and tetrahydroimidazoquinoline compounds that contain urea, thiourea, acylurea, sulfonylurea or carbamate functionality. The compounds are said to be useful as immunomodulators.
U.S. Pat. No. 6,677,349 describes imidazoquinoline and tetrahydroimidazoquinoline compounds that contain sulfonamide functionality at the 1-position. The compounds are said to be useful as immunomodulators.
US-A-2003/0144283 and WO-A-00/76505 describe imidazoquinoline and tetrahydroimidazoquinoline compounds that contain amide functionality at the 1-position. The compounds are said to be useful as immunomodulators.
WO-A-2005/051324 describes imidazo-quinoline, pyridine and naphthyridine rind systems substituted in 1-position with oxime or a special N-oxide functionality. The compounds are said to be useful as immunomodulators.